1. Field of the Invention
This invention relates to a misfire detection system for an internal combustion engine.
2. Description of the Related Art
Japanese Laid-Open Patent Application Hei 5 (1993)-180063 teaches an engine misfire detection system that detects the angular velocity of the engine crankshaft, uses a bandpass filter to extract periodic variation in the angular velocity caused by misfire occurrence, and uses the extracted variation to discriminate whether the engine is misfiring.
In recent years, however, increasingly strict emission controls have created a need to improve misfire detection so as to better hold down increased HC (hydrocarbons) emissions and accelerated catalyst deterioration caused by misfire. Of particular note is that it is nowadays necessary not only to discriminate misfire occurrence but also to identify the cylinder in which it occurred. However, the periodicity of engine speed variation is the same between the case of one cylinder misfiring (single-cylinder misfire) and the case of two cylinders misfiring successively (successive two-cylinder misfire). Therefore, when misfire detection is performed based solely on periodic variation in the angular velocity of the crankshaft as in the case of the prior art, identification of the misfiring cylinder(s) requires various kinds of auxiliary processing for removing frequency components, such a peak holding, absolute value computation and averaging. As a result, much time is required for establishing the detection algorithm.